1. Field of the Invention
This invention relates to an anti-skid control apparatus for a vehicle braking system which can prevent locking of the wheels.
2. Description of the Prior Art
An anti-skid control apparatus for a vehicle braking system is known that includes fluid pressure control valve devices arranged between fluid pressure generating chambers of a tandem master cylinder and the wheel cylinders of the front wheels, respectively, and a control unit receiving outputs of wheel speed sensors for measuring or judging the skid conditions of the front and rear wheels and for generating instructions for controlling the fluid pressure control valve devices.
When the fluid pressure control valve device is provided for each of four wheels (four channels), and their fluid pressures are independently controlled, there is no problem of controlling operation. Or when the fluid pressure control valve device is provided for each of the front wheels, and for both of the rear wheels in common (three channels), there is no problem of controlling operation. In the latter case, the one common fluid pressure control valve device is controlled on the basis of the lower one of the speeds of the rear wheels.
However, in the above cases, three or four fluid pressure control valve devices are used. Accordingly, the whole anti-skid control apparatus is large-sized, and very heavy. Since the fluid pressure control valve device is expensive, the system is expensive.
Accordingly, such apparatus have been proposed that the brake fluid pressures of the front wheels are controlled by the two fluid pressure control valve devices, respectively, and those of the rear wheels diagonally connected to the front wheels are controlled by the same fluid pressure control valve devices, respectively in common with those of the front wheels. The diagonal conduit system or X-type conduit system has been widely used as the counter-measure against the failure of the brake fluid conduits. However, when the brake pressures of the front and rear wheels diagonally connected to each other are controlled in the "select high" manner that the brake pressures are controlled on the basis of the rotational behavior of the one of the front and rear wheels whose frictional coefficient .mu. is higher between its tire and the road, than the other the one wheel on the higher coefficient .mu. road side is controlled so as not to be locked, on the road (split-road) whose frictional coefficients are different between the right and left sides, but the other road on the lower coefficient road side might be locked. At that time, the running or directional stability of the vehicle is lost.
When the brake pressures of the front and rear wheels diagonally connected to each other are controlled in the "select low" manner that the brake pressures are controlled on the basis of the rotational behaviour of the one of the front and rear wheels whose frictional coefficient .mu. is lower between its tire and the road, than the other of them, both of the one and other wheels on the lower and higher coefficients road sides are controlled so as not to be locked, on the split-road.
However, the brake pressure of the other wheel on the higher .mu. side is unnecessarily decreased. Accordingly the braking distance is unreasonably lengthened.